1. Field of the Invention
This invention relates to a method for the production of a gas diffusion electrode. More particularly, this invention relates to a method for the production of a gas diffusion electrode having a platinum group metal deposited thereon by means of an ion-exchange reaction.
2. Description of the Prior Art
Fuel cells of the phosphoric acid type, the solid polymer type, and the like use a gas diffusion electrode having particles of a precious metal such as platinum dispersed on an electroconductive carbon support. In the case of fuel cells of the solid polymer type, there is a particularly strong need for a technology for depositing fine particles of platinum in a highly dispersed state so as to increase the catalyst surface area and obtain an electrode exhibiting high catalytic activity with only a small amount of deposited catalytic metal. As means for obtaining a gas diffusion electrode for a fuel cell having fine platinum particles of a diameter of not more than 3 nm deposited as a catalyst in a highly dispersed state, a method which resides in causing colloidal particles of an oxide of platinum to be deposited on carbon particles (JP-B-61-1,869), a method using a protective colloiding agent (JP-A-56-155,645), and the like have been proposed. These methods adopt a procedure which comprises depositing platinum on a carbon powder, then mixing the platinum-deposited carbon powder with a dispersion of polytetrafluoroethylene (PTFE), and subsequently sheet molding the resultant mixture. The molded sheet thus obtained possesses a water-repelling property. These methods are problematic, however, in the point that, during the sheet molding, the platinum particles agglomerate and lose their catalytic ability or are covered with the carbon particles, PTFE, etc. and prevented from contacting the reaction gas and functioning as a catalyst. As a result, the utilization ratio of catalyst is degraded.
For overcoming this problem, a method which comprises preparatorily forming a sheet electrode possessing a water-repelling property and then impregnating the sheet electrode with, for example, an alcohol solution of chloroplatinic acid (JP-A-05-343,073) has been proposed. With this method, however, it is difficult to attain uniform impregnation of the alcohol solution in the sheet electrode, and the platinum particles are not deposited in a highly dispersed state, because the platinum particles gradually agglomerate.
From the practical point of view, therefore, a strong need is felt for the development of a method enabling easy production of a gas diffusion electrode having particles of a platinum group metal deposited in a highly dispersed state thereon and, at the same time, excelling in catalytic activity.